1. Field of the Invention
The disclosed embodiments relate to thermal imaging systems. In particular, the disclosed embodiments relate to methods and apparatuses for transferring images to media which may be applicable to a direct thermal or thermal transfer processes including dye diffusion.
2. Related Art
A typical thermal imaging system includes a printhead formed by a linear array of thermal elements having density of about 200 to 600 thermal elements per inch. Such a printhead may be used for direct thermal printing or by thermal transfer dye diffusion printing. In direct thermal printing, media having a thermal responsive surface is brought into contact with the printhead and translated over the printhead. While the media is translated over the printhead, thermal elements on the linear array are selectively heated at intervals of about five to twenty-four milliseconds to transfer pixels to the media which correspond to pixels in a desired image. In the dye diffusion process, a donor ribbon and receiver media are translated together over the printhead, the donor ribbon being between the printhead and the receiver media. While the donor ribbon and receiver media are translated over the printhead, the individual thermal elements on the linear array are selectively heated at intervals of about five to twenty-four milliseconds to transfer dye from the donor ribbon to the receiver media to form pixels corresponding to pixels in a desired image.
Each thermal element in either the direct thermal or the dye diffusion process may transfer a pixel image having shades of color or gray between blank (with an unheated thermal element) and opaque (with a fully heated thermal element). Thus, the system selectively heats a thermal element in the linear array to a certain level depending upon the shade of color or gray of the pixel in the desired imaged.
Each of the thermal elements in the linear array includes a resistance and an imaging surface. The imaging system includes a circuit which applies a voltage or current to each of the thermal elements to heat it to a level to transfer a pixel which most closely approximates the shade of color or gray for the pixel in the desired image. A problem arises in existing imaging systems of these types in that, due to manufacturing tolerances, the resistance of the individual thermal elements varies from thermal element to thermal element in the linear array. Since the power applied to each element is related to the resistance associated therewith (i.e., P=V.sup.2 /R and P=I.sup.2 R), the imaging system may apply too little or too much power to a particular thermal element to heat it to a desired level. This results in imaging from thermal elements which may be generally too hot or too cold. Also, compounding with the effects of the differences in resistance from thermal element to thermal element in the linear array, the resistance of each of the thermal elements changes over time as the printhead is used. This causes further distortions in the transferred pixel levels of color or gray.
Additionally, as media is translated over the printhead, thermal elements are repeatedly turned on and off to transfer images to media. In doing so, the imaging system heats the particular thermal elements each time it is to transfer a pixel. Thus, prior to receiving the voltage/current, the imaging surface of any particular thermal element may be cold (e.g., the imaging system has not powered the thermal element for a long time) or the thermal element may be still warm from being heated in the previous five to twenty-four millisecond imaging interval. Thus, in addition to distortions in pixel color or gray level resulting from resistance variances, there may be further distortions due to an historical powering of the thermal elements.